1. Field of the Invention
The invention relates to a device for tissue stimulation in a living organism, of the type having a stimulation pulse generator, fed from a voltage source, for the generation of stimulation pulses, a detector device to detect the reaction of the tissue to the stimulation and a control device which controls the stimulation pulse generator and which, to determine the stimulation sensitivity of the tissue, causes an alteration of the energy of successively generated stimulation pulses until such time as the detector device detects a change from an absence of the reaction to a reaction or vice versa, the stimulation pulses subsequently being generated with an energy which corresponds to the value determined for the stimulation sensitivity plus a safety factor.
2. Description of the Prior Art
A device of the type is known from U.S. Pat. No. 4,878,497 in the form of a heart pacemaker by which the heart of a patient is excited with the aid of stimulation pulses. In this case, a contraction takes place as reaction of the heart only in circumstances in which the energy of the stimulation pulses exceeds a specified stimulation threshold which is evident from the stimulation sensitivity of the tissue of the heart. In order to keep the energy consumption at a low level when using the known heart pacemaker, the stimulation energy is matched to the alterable stimulation sensitivity by determining the stimulation sensitivity of the tissue of the heart at specified time intervals and subsequently setting the stimulation energy to a value which corresponds to the energy value (stimulation threshold) determined for the stimulation sensitivity plus a safety factor. In order to determine the stimulation sensitivity, the energy of the stimulation pulses is increased successively in steps, proceeding from a value which is below the stimulation threshold, until such time as a reaction of the tissue of the heart to the stimulation is detected by the detector device. After the determination of the stimulation sensitivity and the setting, matched thereto, of the stimulation energy, a regular check is made, until the next procedure of determination by detection of the heart reaction, as to whether the set stimulation energy exceeds the stimulation threshold of the tissue of the heart. If this is not the case, the stimulation energy is increased. In this case and in the determination of the stimulation sensitivity, the alteration of the energy of the stimulation pulses takes place, proceeding from an instantaneous pulse duration, by first increasing the pulse amplitude (electrical current strength) in steps to a specified value, and subsequently the pulse duration is increased in steps and finally, on reaching a maximum pulse duration, the pulse amplitude is increased to a maximum value.
German AS 2,254,928 discloses a heart pacemaker in which, in order to determine the stimulation sensitivity of the tissue of the heart, the pulse amplitude of successive stimulation pulses is reduced in steps, proceeding from a value which is above the stimulation threshold of the tissue of the heart, until such time as an absence of the reaction of the heart is detected.
As is known, among the stimulation pulses which, in the case of a specified stimulation sensitivity of a tissue to be stimulated, are still just capable of triggering a stimulation, those stimulation pulses which have a shorter pulse duration and a greater pulse amplitude exhibit a lower current consumption than stimulation pulses which have a smaller pulse amplitude and a longer pulse duration. Since, especially in the case of implanted devices, the stimulation energy is as a rule drawn from a battery as voltage source, the aim is to achieve the lowest possible drain of charge from the voltage source in conjunction with the generation of the stimulation pulses. The desire also exists, especially on the part of physicians, to be able to express the safety factor between the stimulation threshold of the tissue and the stimulation energy set as a function thereof in the form of an electric voltage, i.e. in volts.